1. Field of the Invention
This invention relates to a technique for depositing metals from their aqueous solutions by reduction and, more particularly, to electroplating, electroless plating, manufacture of fine metal particles, and/or recovery of metals.
2. Description of the Prior Art
Techniques for obtaining metals by reductive deposition from the solutions in which the metals are dissolved in the form of ions (simple ions and complexes) are in use for refining and recovery of metals, plating, and manufacture of fine metal particles and so on. In the area of those techniques, the development of new process starts with the preparation of a solution capable of dissolving an objective metal as ions and sustaining operation for long. The simplest practice at the start is dissolving a soluble salt of the metal. For this purpose various salts such as sulfate, chloride, nitrate, sulfonate, borofluoride, sulfamate, phosphate and so on have been employed. Where such a simple salt is not soluble or can seldom form a solution capable of yielding a metal coating or metal particles with satisfactory physical properties, a complex, instead of simple ions, is used in preparing a solution. Cyanide is deemed typical of the complexing agents that have long been used in providing stable solutions. Complexes have found use in other diversified applications. When alloy coatings or alloy particles are to be obtained, a complex is utilized to bring the deposition potentials of two or more metals close to one another so as to make an alloy of desired composition. A complex controls the balance between the nucleation at the time of metal deposition and the growth of crystals from the nuclei to afford a metal or alloy of intended physical properties. These modern applications have combined with the trend for restricting the use of cyanide because of its environmental hazard to encourage the development and utilization of various complexing agents suited for the diversified applications. Examples of complexing agents abound; inorganic complexing agents including halogens such as iodide ion and bromide ion, condensed phosphate complexing agents such as pyrophosphate ion and tripolyphosphate ion, oxy-acid ions of sulfur such as thiosulfate ion and sulfite ion; and organic complexing agents including carboxylic or oxycarboxylic acid ones such as citric, tartaric, and gluconic acids, aminecarboxylate ones such as EDTA, DTPA, IDA, and NTA, and compounds containing nitrogen or sulfur or both such as urea, thiourea, succinimide, hydantoin, and various mercaptocarboxylic acids.
These varied complexing agents may be used singly or in combination to prepare aqueous solutions for the reductive deposition of metals. However, many of the resulting solutions are still unsatisfactory from the industrial viewpoint, not merely in achieving the above purpose, or stabilization of the bath, but also in the control of deposition potential, crystal growth, and other factors. As regards electroplating and electroless plating of gold, for example, it has been proposed and adopted in some sector of the industry to use sulfurous acid, thiosulfuric acid, mercapto-carboxylic acids, etc. so as to prepare cyanide-free plating baths. The disadvantages of the proposed attempts are short bath life and the possibility of contamination of sulfur in the plating deposits. Some attempts have also been made to add a slight amount of nickel, cobalt, antimony, tin or other similar metal to the plating solution so that codeposition can enhance the hardness of the gold plate. Nevertheless, the addition of such a metallic element results in weaker stability of the solution. In electroplating of silver too, there is a demand for cyanide-free baths, and solutions using iodine, succinimide, or the like as a complexing agent have met partial commercial acceptance. Here again the stability, adhesion, and other problems of the baths have hampered the widespread adoption of such solutions. In electroless plating of palladium, ammine complexes are used in the plating solutions, but the baths have a stability problem. Further, for the electroplating of silverpalladium alloy that exhibits outstanding electrical characteristics, research has been made on the use of a variety of complexing agents including ammonia, nitrite, ethylenediamine, pyrophosphate, glycine, halogens, and thiocyanate. No industrially satisfactory solution has, however, been developed yet which can find wide acceptance. Silver-tin alloy coating has been deemed promising because of the favorable contact electric resistance characteristics and resistance to tarnishing inherent to silver. Nevertheless, it has not come into extensive industrial use since the wide discrepancy between the deposition potentials of the two metals presents difficulties in developing a solution with excellent stability and ability of easily forming a coating of desirable composition.
In order to settle the principal problem of providing a stable solution and then settle various concomitant problems, e.g., Japanese Patent Publn Kokai No. 63-259093 discloses the utilization of sulfonated phosphine in a solution for recovering rhodium electrolytically. Patent Publn Kokai No. 8-257418 employs sulfonated phosphine in an electrochemical process for producing a catalyst composed mainly of a transition metal and phosphine. Patent Publn. Kokai No. 8-225985 teaches the use of lower alkyl phosphines, cycloalkylphosphines, triphenylphosphine, and other phosphines, alkyl phosphonic acids, hydroxyalkane diphosphonic acids and the like to restrict the displacement deposition of bismuth as well as the variation in the bismuth content in the deposit from a bismuth-tin alloy plating bath.
However, the lower alkyl, cycloalkyl, phenyl phosphines and the like taught in Patent Publn Kokai No. 8-225985 are not, in essence, water soluble and present tremendous difficulties in use at the concentrations in the range of 1-500 g/l as explained in the specification for the application. In addition, the introduction of sulfonic acid group to impart water solubility makes the sulfonated aryl phosphine so expensive that the industrial application for the plating bath is limited to particularly high-priced metals such as rhodium.